This invention relates to devices for damping perturbations on marine structures, and particularly devices directed to damping low frequency perturbations due to wind, water, and other marine forces.
It is known to use a partially filled tank of liquid to damp natural (fundamental) frequency oscillations of a fixed marine structure. Such a device is disclosed in Vandiver et al., U.S. Pat. No. 4,226,554, which is herein incorporated by reference. The figures of Vandiver et al. show a non-toroidal rectangular-shaped tank mounted on the structure. The tank of Vandiver et al. is used for damping oscillations in a structure fixed to a support mounted on the sea floor, as shown in FIG. 1. Such fixed structures have a high stiffness associated with their dominant modes of motion, and thus behave when oscillating like stiff springs. As such, Vandiver et al. showed that their motion dynamics can be described using linear models.
The dynamics of compliant (i.e., free floating or tethered) structures are somewhat different. Compliant structures have low stiffness associated with their motions in the horizontal plane, and thus behave more like soft springs. As such, a linear model of behavior in the case of compliant structures would ignore important non-linear effects. For example, in compliant structures, there is a significant non-linear coupling between vertical (or heave) and horizontal (or sway) excitations on the structure due to, e.g., wind, wave action, or underwater seismic disturbances. This coupling leads to a time-dependent stiffness term in the motion equation for the horizontal plane. The mathematical model for motions in the horizontal plane is thoroughly described in R. Rainey, "Parasitic Motions of Offshore Structures," Transactions of the Royal Institutions of Naval Architects 177 (1982), which is herein incorporated by reference. The model resembles the Mathieu equation, the solutions of which show responses at frequencies lower than the natural (of fundamental) frequency of the structure. These frequencies are known as subharmonic resonances, and are very difficult to design against. Most marine structures are dynamically designed so that their natural frequencies are outside the range of the excitation frequencies of the sea. However, such designs do not correct for subharmonic resonances. In the open ocean, particularly in very deep waters (i.e., for a structure the size of an offshore platform, depths of greater than 2500 ft.), there exist swells and other low frequency excitations over and above the dominant natural wave frequency. In addition, there exist drift forces due to group waves which create very low frequency excitations in the horizontal plane in compliant structures. In free-floating structures, these forces can cause undesirable shifts in position. In tethered structures, these forces can cause dynamic stressing and fatigue of the tethers.